1. Field of the Invention
The present invention relates to a microcontroller unit (MCU), and more particularly to a power noise preventing circuit for a MCU.
2. Discussion of the Background
Preventing an erroneous operation due to noise is one of the basic requirements of many systems. A microcontroller unit (MCU) is an integrated circuit with the least resistance to noise, and accordingly, preventing power noise is more seriously required.
FIG. 1 is a block diagram of a related art power noise preventing circuit for the MCU. The related art power noise preventing circuit for the MCU includes a power fail detecting circuit 10 for enabling a power fail signal Power.sub.-- Fail when power is changed to a voltage with which the MCU is not able to operate. The related art power noise preventing circuit for the MCU further includes a resetting circuit 20. The resetting circuit 20 resets the MCU in accordance with the enabled power fail signal.
When power is changed to the voltage at which the MCU does not operate, e.g., when the power noise is received, the power fail detecting circuit 10 enables the power fail signal Power.sub.-- Fail. The resetting circuit 20 resets the MCU in accordance with the enabled power fail signal Power.sub.-- Fail to prevent the erroneous operation of the MCU caused by the noise. Meanwhile, a system clock generating circuit 30, as shown in FIG. 2, receives a basic clock signal ICLK from an oscillator and generates a system clock signal SCLK, which is needed for an internal circuit.
However, the system clock generating circuit 30 continuously outputs the system clock signal SCLK to the internal circuit even though the power noise is inputted to the MCU. Accordingly, an erroneous operation of the system can occur when the power fails. In addition, when the erroneous operation of the system is caused by the power fail, it is difficult or impossible for the system to return to a normal state. Therefore, the MCU is unconditionally reset when the power noise is inputted.
As described above, the related art power noise preventing circuit has various disadvantages. When the MCU is reset, the system using the MCU also is reset to an initial state. For example, when the MCU is reset, a TV is turned off, a washing machine stops operating and returns to an initial state, or a medical appliance that is being operated is suspended. As a result, the related art power noise preventing circuit for the MCU may result in a dangerous situation because of a sudden operational failure of appliances. Additionally, when restarting the operation of the MCU and the system using the MCU in the initial state or in a suspended state, various related systems should be restarted or reset, thus causing inconvenience.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.